Desynchronization of pedal motion: Crystallographic and theoretical study of (E)-4-[(4-ethylphenyl)diazenyl]-2-methylphenol

Abstract

The single crystal X-ray diffraction analysis of the title compound, C15H16N2O, reveals that its molecules exhibit whole-molecule disorder at both crystal lattice sites due to pedal motion in solid state. The compound crystallizes in the monoclinic space group P 2(1)/c with a = 20.5504(14) angstrom, b = 10.8887(5) angstrom, c = 12.0191(8) angstrom and beta = 96.927(5)degrees. While major pedal conformers of the compound in solid state are stabilized by intermolecular O-H center dot center dot center dot N type hydrogen bonds leading to the formation of C(7) chains at Site 1 and C(8) chains at Site 2 along [0 1 0] axis, C-H center dot center dot center dot pi type intermolecular interactions between major and minor conformers also serve to stabilize minor pedal conformers. An interesting feature about the crystal structure is that pedal conformers at Site 1 have two different occupancy factors arising from desynchronization of pedal motion along [2 1 0] direction in crystal phase. Quantum chemical calculations at the B3LYP/6-31++G** level suggest that the desynchronization of pedal motions make more unstable pedal conformers at Site 1 than those at Site 2.